Currently, a white LED becomes more and more popular for the backlighting unit (BU) of various TFT LCD devices, such as LED TV and LED monitor. The edge light design of the white LED has the following advantages compared with the traditional CCFL BU:                1. Super slim structure. The total thickness of the display panel including front bezel, TFT panel, backlight and back cover will be much thinner than that of the RGB LED panel and the CCFL BU panel.        2. More uniform color range across the screen. The LED light can be arranged either horizontally or vertically.        3. More environmental friendly.        
But there are two disadvantages of the white LED BU compared with CCFL BU:                1. The color gamut of the white LED is not as wide as CCFL BU in red and green color wavebands. Color rendering index, Ra, is 75 for Blue LED+Yellow Phosphor type, while CCFL's Ra is over 80.        2. Brightness and energy efficiency is lower due to phosphor conversion, stokes shift and self absorption.        
The current TFT makers around the world are trying to redesign the absorptions and the transmissions of color filter arrays via adjusting R.G.B color coordinates to fit the LED BU. Obviously, such designs which renders more absorption in the blue color and less absorption in green and red will increase the energy consumption and decrease the color quality and the brightness of the display. In other word, the color balancing is actually in the cost of further reduction of the brightness of the LED BU.
The question is how to achieve better brightness and color balance with a less power consumption than that of the traditional CCFL backlight?
The solution is to use a special brightness enhancement film, which is capable of matching the spectrum of the white LED so as to enhance the brightness and the Color quality of the LED TV/monitor.
Cholesteric liquid crystal polymer (CLCP) film is characterized by the fact that it selectively reflects the incoming light and turns out a narrow band circular polarization to the front viewer. The CLCP film is utilizing “Bragg reflection”, one of the intrinsic properties of cholesterics. In Bragg reflection, only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer. The remaining spectra of the incoming light, however, including the 50% opposite-handed circular polarization and the same-handed out-off Bragg reflection wave band will pass through the film. Theoretically, the reflective component is narrow band circular polarization while the transmissive one is broadband elliptical polarization.
A broadband reflection can be also realized by means of changing the helical structure of the CLCP film.
The cholesteric broadband polarizer (BBP) was first disclosed, to the knowledge of the applicant, in the article “Cholesteric Color Filter Made From Cholesteric LC Silicones”, published May 15, 1990 (SID 90 Digest. 111). The paper describes experiments concerning the construction of broadband polarizer by combining layers of cholesteric LC-silicones of different reflection wavelength. Five LC-silicon layers were stacked together and the circular polarization was observed from 430 nm to 670 nm. The ellipticity spectra for the combined layers were also calculated from the spectra of each single layer. The good agreement with the observed spectra clearly demonstrates the conservation of circular polarization by transmitting light through cholesteric layers. For that reason it is possible to arrange LC-layers with different Bragg reflection wavelengths to get broadband filters without loss of circular polarization.
The European Patent Application 94200026.6 with the title of “Cholesteric Polarizer and Manufacture Thereof”, published Jul. 20, 1994 and assigned to Philips Electronics, N.V. of Eindhoven, Netherlands (the “Philips reference”) introduces a method to make a single layer CLCP film having broadband reflection and transmission characteristics. The Philips disclosure requires adding a UV dye into CLCP mixture so that the pitch of the CLCP material changes linearly from its maximum value at one film surface to its minimum value at the other film surface. The CLCP material is formed from two polymerizable chiral and nematic monomers, each of which has a different reactivity. During polymerization of the mixture by means of UV exposure, a linear variation in UV light intensity is to be preferentially incorporated into the least reactive monomer to occur at the location of the highest radiation intensity. As a result, at least one concentration gradient of free monomers is formed during polymerization, causing the monomer to diffuse from locations with a low monomer concentration to the location with a high concentration. The monomers of high reactivity diffuse to the locations where the radiation intensity is the highest. As a result, the composition of the material varies in a direction transverse to the surfaces of the film such that a linear variation in the pitch of the molecular helices results in the layer formed by the polymer. The liquid crystal material is distributed linearly across the thickness of the film. This variation in pitch provides the optically active layer with a bandwidth proportional to the variation in the pitch of the molecular helices.
An article “From Selective to Wide-band Light Reflection: a Simple Thermal diffusion in a Glassy Cholesteric Liquid Crystal”, published Dec. 17, 1998, Physical Journal B, France, introduces a method to fabricate a wide-band circular polarizer. The method relates to a spontaneous diffusion of monomers into a polymerizable CLCP film and then following a UV polymerization. The fabrication is carried out by depositing a film of reactive monomers on the surface of a polymerized film of CLCP material. The diffusion of monomers into the CLCP film layer causes a concentration gradient in the layer before diffusion is halted. As a result, the original CLCP material swells slightly causing an increase in pitch of the molecular helices. This provides a concentration gradient which, in turn, results in a “linear variation” in pitch across the film thickness. Polymerization of the layer by UV light exposure halts diffusion providing a broadband polarizer.
U.S. Pat. No. 6,532,049 with the title of “Circularly Polarizing Reflective Material. Having Super Broad-band Reflection and Transmission Characteristics and Method of Fabricating and Using Same in Diverse Applications” published Mar. 11, 2003 and assigned to Reveo, Inc. of Elmsford N.Y. (the “Reveo reflerence”), introduces a method for fabricating a broadband circularly polarizing material. According to the method, a CLCP material is mixed with non-cross linkable liquid crystal material, a photoinitiator and a chiral additive at a temperature, which maintains the mixture in a liquid crystal state. While being heated, the mixture is subjected to UV light radiation for a time and at an intensity sufficient to polymerize the CLCP material or the liquid crystal material or both. Such polymerization occurs in a non-linear fashion, thereby resulting in a non-linear distribution of the polymer and the liquid crystal material across the During polymerization, phase separation takes place. The segregation rate of the liquid crystal material is designed to be greater than the polymerization rate of the CLCP material being polymerized. Thus, the liquid crystal material segregates and diffuses to sites of enlarged pitch in the CLCP material from sites of shrunken pitch in the CLCP material. Consequently, an exponentially distributed pitch is generated from one surface to the other of the CLCP film.
In the U.S. Pat. No. 7,095,466 with the title of “Diffusively Reflective Circular Polarizer formed By thermo Phase Separation,” the applicant introduces a fabrication method of thermo phase separation to convert the CLCP film from narrow band planar structure into broadband microchip structure, herein incorporated by reference.
Within the above-mentioned prior art, all the reflective polarizers reported are designed for the full spectrum broadband applications, which could be used for the traditional CCFL backlighting unit. However, the production process is normally slow and the LCP film is relatively thick to achieve a sufficient bandwidth. Therefore, the limitations of production throughput and material cost have remarkably hindered its application as the brightness enhancement film.